/******************************************************************************
 * Copyright 1998-2019 Lawrence Livermore National Security, LLC and other
 * HYPRE Project Developers. See the top-level COPYRIGHT file for details.
 *
 * SPDX-License-Identifier: (Apache-2.0 OR MIT)
 ******************************************************************************/

#ifdef __cplusplus
#define REGISTER 
#else
#define REGISTER register
#endif

/*
 * distributed_qsort.c:
 * Our own version of the system qsort routine which is faster by an average
 * of 25%, with lows and highs of 10% and 50%.
 * The THRESHold below is the insertion sort threshold, and has been adjusted
 * for records of size 48 bytes.
 * The MTHREShold is where we stop finding a better median.
 */

#include <stdlib.h>			/* only for type declarations */
#include "_hypre_utilities.h"

#define		THRESH		4	/* threshold for insertion */
#define		MTHRESH		6	/* threshold for median */

static HYPRE_Int (*qcmp) (char*,char*);		/* the comparison routine */
static HYPRE_Int qsz;			/* size of each record */
static void qst(char *, char *);

static HYPRE_Int thresh;		/* THRESHold in chars */
static HYPRE_Int mthresh;		/* MTHRESHold in chars */



/*
 * hypre_tex_qsort:
 * First, set up some global parameters for qst to share.  Then, quicksort
 * with qst(), and then a cleanup insertion sort ourselves.  Sound simple?
 * It's not...
 */

void
hypre_tex_qsort(char* base,HYPRE_Int n,HYPRE_Int size, HYPRE_Int (*compar) (char*,char*))
{
    REGISTER char *i;
    REGISTER char *j;
    REGISTER char *lo;
    REGISTER char *hi;
    REGISTER char *min;
    REGISTER char c;
    char *max;

    if (n <= 1)
	return;
    qsz = size;
    qcmp = compar;
    thresh = qsz * THRESH;
    mthresh = qsz * MTHRESH;
    max = base + n * qsz;
    if (n >= THRESH)
    {
	qst(base, max);
	hi = base + thresh;
    }
    else
    {
	hi = max;
    }
    /* First put smallest element, which must be in the first THRESH, in the
       first position as a sentinel.  This is done just by searching the
       first THRESH elements (or the first n if n < THRESH), finding the min,
       and swapping it into the first position. */
    for (j = lo = base; (lo += qsz) < hi;)
    {
	if ((*qcmp) (j, lo) > 0)
	    j = lo;
    }
    if (j != base)
    {				/* swap j into place */
	for (i = base, hi = base + qsz; i < hi;)
	{
	    c = *j;
	    *j++ = *i;
	    *i++ = c;
	}
    }
    /* With our sentinel in place, we now run the following hyper-fast
       insertion sort.  For each remaining element, min, from [1] to [n-1],
       set hi to the index of the element AFTER which this one goes. Then, do
       the standard insertion sort shift on a character at a time basis for
       each element in the frob. */
    for (min = base; (hi = min += qsz) < max;)
    {
	while ((*qcmp) (hi -= qsz, min) > 0);
	if ((hi += qsz) != min)
	{
	    for (lo = min + qsz; --lo >= min;)
	    {
		c = *lo;
		for (i = j = lo; (j -= qsz) >= hi; i = j)
		    *i = *j;
		*i = c;
	    }
	}
    }
}



/*
 * qst:
 * Do a quicksort
 * First, find the median element, and put that one in the first place as the
 * discriminator.  (This "median" is just the median of the first, last and
 * middle elements).  (Using this median instead of the first element is a big
 * win).  Then, the usual partitioning/swapping, followed by moving the
 * discriminator into the right place.  Then, figure out the sizes of the two
 * partions, do the smaller one recursively and the larger one via a repeat of
 * this code.  Stopping when there are less than THRESH elements in a partition
 * and cleaning up with an insertion sort (in our caller) is a huge win.
 * All data swaps are done in-line, which is space-losing but time-saving.
 * (And there are only three places where this is done).
 */

static void qst(char *base, char *max)
{
    REGISTER char *i;
    REGISTER char *j;
    REGISTER char *jj;
    REGISTER char *mid;
    REGISTER HYPRE_Int ii;
    REGISTER char c;
    char *tmp;
    HYPRE_Int lo;
    HYPRE_Int hi;

    lo = max - base;		/* number of elements as chars */
    do
    {
	/* At the top here, lo is the number of characters of elements in the
	   current partition.  (Which should be max - base). Find the median
	   of the first, last, and middle element and make that the middle
	   element.  Set j to largest of first and middle.  If max is larger
	   than that guy, then it's that guy, else compare max with loser of
	   first and take larger.  Things are set up to prefer the middle,
	   then the first in case of ties. */
	mid = i = base + qsz * ((unsigned) (lo / qsz) >> 1);
	if (lo >= mthresh)
	{
	    j = ((*qcmp) ((jj = base), i) > 0 ? jj : i);
	    if ((*qcmp) (j, (tmp = max - qsz)) > 0)
	    {
		j = (j == jj ? i : jj);	/* switch to first loser */
		if ((*qcmp) (j, tmp) < 0)
		    j = tmp;
	    }
	    if (j != i)
	    {
		ii = qsz;
		do
		{
		    c = *i;
		    *i++ = *j;
		    *j++ = c;
		} while (--ii);
	    }
	}
	/* Semi-standard quicksort partitioning/swapping */
	for (i = base, j = max - qsz;;)
	{
	    while (i < mid && (*qcmp) (i, mid) <= 0)
		i += qsz;
	    while (j > mid)
	    {
		if ((*qcmp) (mid, j) <= 0)
		{
		    j -= qsz;
		    continue;
		}
		tmp = i + qsz;	/* value of i after swap */
		if (i == mid)
		{		/* j <-> mid, new mid is j */
		    mid = jj = j;
		}
		else
		{		/* i <-> j */
		    jj = j;
		    j -= qsz;
		}
		goto swap;
	    }
	    if (i == mid)
	    {
		break;
	    }
	    else
	    {			/* i <-> mid, new mid is i */
		jj = mid;
		tmp = mid = i;	/* value of i after swap */
		j -= qsz;
	    }
    swap:
	    ii = qsz;
	    do
	    {
		c = *i;
		*i++ = *jj;
		*jj++ = c;
	    } while (--ii);
	    i = tmp;
	}
	/* Look at sizes of the two partitions, do the smaller one first by
	   recursion, then do the larger one by making sure lo is its size,
	   base and max are update correctly, and branching back. But only
	   repeat (recursively or by branching) if the partition is of at
	   least size THRESH. */
	i = (j = mid) + qsz;
	if ((lo = j - base) <= (hi = max - i))
	{
	    if (lo >= thresh)
		qst(base, j);
	    base = i;
	    lo = hi;
	}
	else
	{
	    if (hi >= thresh)
		qst(i, max);
	    max = j;
	}
    } while (lo >= thresh);
}
